Accuracy of Body Mass Index

Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population


Body mass index (BMI) is the most widely used measure to diagnose obesity. However, the diagnostic accuracy of BMI to detect excess in body adiposity is largely unknown.


A cross-sectional design of 13,601 subjects (age 20–79.9 years; 48% men) from the Third National Health and Nutrition Examination Survey. Bioelectrical impedance analysis was used to estimate body fat percent (BF %). We assessed the diagnostic performance of BMI using the World Health Organization reference standard for obesity of BF % > 25% in men and > 35% in women. We tested the correlation between BMI and both, BF % and lean mass by sex and age groups.


BMI-defined obesity (≥ 30 kg/m2) was present in 21% of men and 31% of women, while BF %-defined obesity was present in 50% and 62%, respectively. A BMI ≥ 30 had a high specificity (95% in men and 99% in women), but a poor sensitivity (36% and 49 %, respectively) to detect BF %-defined obesity. The diagnostic performance of BMI diminished as age increased. BMI had a good correlation with BF % in men (R2 = 0.44) and women (R2 = 0.71), but also with lean mass (R2 = 0.50 and 0.55, respectively).


Despite the good correlation between BMI and BF %, the diagnostic accuracy of BMI to diagnose obesity is limited, particularly for individuals in the intermediate BMI ranges. A BMI cut-off of ≥ 30 kg/m2 has a good specificity but misses more than half of people with excess fat. These results help to explain the U and J-shape association between BMI and outcomes.

Could this have something to do with the recent “revelation” that obesity is not generally bad for health? I think so. BMI-defined obesity missed many of the BF-defined obese. In other words, many of those with lower BMIs are actually obese, thus their health is statistically the same as the BMI-defined obese.


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Halffull says January 5, 2013

I looked at the study about “obesity is not bad for health”… it actually says that being “overweight” is not bad for health, obesity itself is correlated with negative health outcomes in the same study. I suspect this is getting misinterpreted because colloquially the two are interchangeable.

dearieme says January 7, 2013

“the diagnostic accuracy of BMI to detect excess in body adiposity is largely unknown”: i.e. it’s all been a matter of faith, not science. Moreover, there’s nothing in that summary that implies that there’s any good evidence for the criterion that BF % > 25% in men is bad for their health. It’s obviously implausible since it makes no allowance for age.

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